sideload cannot be used when using deadman for scheduled maintenance

Hi,
How do I use deadman in combination with sideload for doing scheduled maintenance? The problem is when using deadman after sideload, the added fields are no longer available for further processing as it uses [stats()](https://docs.influxdata.com/kapacitor/v1.5/nodes/eval_node/#deadman) [internally](https://github.com/influxdata/kapacitor/blob/e6bc51b8447de450c3f6fc0f6e47b6a0987ce5b6/pipeline/node.go#L280). So far the only way I've tried is to use the equivalent script describe in the docs
and insert sideload in between derivative and alert().

Regards,

Jeff

kapacitor version
Kapacitor OSS 1.5.4 (git: HEAD 1f648f85772efe222a3853fe1a0d9ef88854a8c1)

Given the ff:
```
stream
|from()
.database('telegraf')
.measurement('cpu')
|sideload()
.source('file:///data/scheduled-maintenance')
.order('hosts/{{.host}}.yml' , 'hostgroups/{{.hostgroup}}.yml')
.field('maintenance', FALSE)
.tag('testtag', 'testtag')
|deadman(0.0,20s, lambda: !"maintenance" )
```

Result: err="reference \"maintenance\" is missing value"

Equivalent script:
```
|derivative('emitted')
.unit(10s)
.nonNegative()
|sideload()
.source('file:///data/scheduled-maintenance')
.order('hosts/{{.host}}.yml', 'hostgroups/{{.hostgroup}}.yml')
.field('maintenance', FALSE)
.tag('testtag', 'testtest')
|alert()
.crit(lambda: !"maintenance" AND "emitted" <= 0)
```